1. Field of the Invention
The present invention relates to the hydraulic control system of a 4-speed automatic transmission for a vehicle and, more particularly, to such a hydraulic control system for a four-speed automatic transmission which can enhance shift quality by alleviating shock caused by gear shifting and can improve responsiveness with respect to the gear shifting.
2. Description of Related Art
In general, an automatic transmission provides a necessary gear ratio to operate the vehicle under wide range of speeds and loads. It does this with a minimum amount of effort on the part of the driver. That is, automatic upshifts and downshifts are a convenience for the driver because a foot-operated clutch is not required to shift gears and because the vehicle can be brought to a stop without the use of a clutch and without shifting the transmission into neutral.
A conventional automatic transmission for a vehicle comprises a torque converter having an impeller, a turbine and a stator, a gear train connected to the torque converter to provide various forward speed ranges and reverse, a plurality of friction members such as disc clutches, one-way clutches which control gear action, and a hydraulic control system for controlling the operation of the friction members.
A hydraulic pressure control system of a 4-speed automatic transmission vehicle is described in U.S. Pat. No. 4,506,563. Since this system can not provide a skip shifting, e.g. from the fourth speed to the second speed, responsiveness with respect to a gear shift is retarded.
Additionally, the system described in U.S. Pat. No. 4,506,563 is designed such that only two predetermined modes of pressures, that is, pressure of a drive "D" range and pressure of a reverse "R" range, are fed to the pressure line, thereby occurring drive loss of the hydraulic pump and deteriorating the fuel consumption ratio.
Further, since the system described in U.S. Pat. No. 4,506,563 is designed such that the hydraulic pressure are simultaneously fed to a front clutch and a release chamber of a kick-down servo band brake, shock caused by gear shifting, especially, down shifting from a third speed ratio to a second speed ratio occurs.